Owing to the relatively high toxicity of mercury, especially organically bound mercury which is taken up directly or indirectly via the food chain by human beings, there are relatively strict limits to the legally permissible emissions of mercury from combustion plants and power stations.
Mercury and mercury compounds are essentially ubiquitous and more or less present in all organic or fossil fuels. Combustion of fossil fuels mobilizes mercury and mercury is released in gaseous form via the offgas and also in dissolved form via the wastewater streams or in solid form via sludges into the environment. This results in the necessity of keeping mercury out of the flue gas from combustion plants.
Mercury is present in essentially two chemical forms, namely as elemental mercury and as oxidized divalent mercury, in the flue gas from combustion plants and especially in the flue gas from coal-fired power stations. Depending on the mercury, halogen and sulfur content of the fuel used and the process steps and also conditions in the flue gas treatment up to release into the environment, a particular ratio of the proportions of oxidized and elemental mercury is established. The oxidation state of the mercury and the ratio of the species is critical to the type and effectiveness of a mercury removal technology. In contrast to elemental mercury, oxidized mercury is water-soluble and can therefore be scrubbed out from the flue gas by means of a wet gas scrub, for example by means of a wet flue gas desulfurization plant, and removed from the process by means of adsorbents or by means of ion exchangers or organic precipitants and flocculants and converted into a solid which can be disposed of in a landfill.
However, this procedure is not suitable for removing elemental mercury from the flue gas. If the proportion of elemental mercury in the flue gas is high, the mercury can be converted by, for example, addition of oxidants into a soluble oxidized species which can then be removed from the flue gas in a downstream wet flue gas scrub.
In the case of fluidized-bed boilers with desulfurization by the dry additive process and other plants without a wet scrub, it is prior art to blow adsorbents in undoped form or doped with bromides or with sulfur into the flue gas stream and mechanically separate the adsorbents loaded with mercury from the flue gas stream, for example by means of fabric filters or by means of electrostatic dust precipitators.
These processes of dry entrained-flow adsorption using carbon-containing adsorbents incurs a certain risk to operational safety of the adsorption plants because of the fundamental ignition tendency of carbon-containing adsorbents. For this reason, numerous efforts are known to have been made to replace carbon-containing adsorbents by other adsorbents, for example by zeolites.
Despite all efforts to dispense with organic adsorbents for dry offgas purification, these organic adsorbents have the advantage of an extraordinarily high adsorption capability.
A process for dry purification of offgases from thermal processes using dry pulverulent sorbents based on brown coal coke is known, for example, from DE 199 40 683 A1. The sorbents are introduced in powder form into the offgas stream and entrained by the latter in such a way that adsorption of the harmful constituents of the offgas occurs on the sorbents present in the form of a dust cloud in the offgas stream. In order to satisfy requirements in respect of fire and explosion protection, the process according to DE 199 40 683 A1 provides for zeolites or zeolite-containing mineral rock to be employed as surface-active constituent in admixture with brown coal coke as adsorbent and for the mixing ratio between the brown coal coke and zeolite to be set as a function of the type, composition and amount of the process dust present in the offgas in such a way that it is ensured that the solids are made inert.
As an alternative to the use of brown coal coke as adsorbent, the use of activated carbon is in principle also known for dry offgas purification, for example from DE 40 34 498 A1. Activated carbon is produced in a complicated manner from coconut shells by pyrolysis and is therefore expensive, so that efforts were always made in the prior art to keep the use of activated carbon as sparing as possible.
A process for removing mercury from flue gases from combustion plants is known from DE 39 41 894 A1. Further prior art is known from the documents U.S. Pat. Nos. 5,811,066, 7,722,843 B1, 6,818,043 B1 and DE 10 2007 042 297 A1.